1. Field of the Invention
The present invention pertains to fluid pressure responsive apparatus and more particularly concerns a vibrating diaphragm sensor apparatus for converting fluid pressure magnitude directly into an electrical signal whose frequency varies as a function of that applied fluid pressure.
2. Description of the Prior Art
The immediate prior art vibrating diaphragm fluid pressure sensor is that of the R. H. Frische U.S. Pat. No. 3,456,508, issued July 22, 1969, and assigned to Sperry Rand Corporation. In this prior Frische patent, antecedent concepts for pressure sensors generally unsuited for application in aircraft digital air data and altitude sensing systems are also discussed. The device of the former Frische patent overcomes limitations of such prior art transducers by use of a simple, flat diaphragm not requiring association with a vibrating wire. Further, it directly measures gas pressure rather than gas density with the change in the diaphragm vibrating frequency resulting from changes in the mechanical spring rate of the diaphragm as a function of fluid pressure loading. Most important, the device has an output frequency variation substantially greater than prior art devices over pressure ranges of interest particularly in air data and altitude sensing systems.
In more particularity, the device of the prior Frische patent utilizes a pressure chamber having a wall defined by a flat diaphragm uniformly restrained at its periphery and subjected to fluid pressure differences between one side and the other. The diaphragm becomes stiffer in a non-linear fashion the farther it is deformed from its flat or unstressed position by the varying pressure of fluid acting on one of its sides. Thus, the diaphragm deforms easily for the first several increments of applied fluid pressure but, as the pressure is progressively increased, additional deformation progressively diminishes. The diaphragm may properly be considered as a spring-mass mechanical system, and it can therefore be driven at a characteristic resonant frequency which is a function of its effective mass and spring stiffness. As the diaphragm is deformed to a lesser or greater degree by changes in gas pressure, its stiffness changes and its mechanically resonant frequency changes as a true function of applied pressure. Thus, the flat diaphragm system provides the desired pressure-to-frequency conversion characteristic needed for digital pressure measurement applications.
The vibrating diaphragm sensor of the prior Frische patent has been widely accepted as a reliable and accurate means for measuring gas pressure, many problems associated with the structural design of the vibrating diaphragm itself and with thermal and vibration isolation from the environment having been generally resolved. The pressure chamber geometry is determined largely by factors inherent in the design and successful manufacture of the vibrating diaphragm. However, it is found that the vibrating nature of the device may give rise to acoustic waves within the interior of its gas chamber or within the pneumatic lines coupled to the sensor which waves, under certain circumstances, interfere with the degree of precision of pressure measurement obtainable by the device. Inherently, the vibrating diaphragm pressure sensor operates over a frequency range dependent upon the range of gas pressures to be measured, and therefore the acoustic waves generated are of varying frequencies and amplitudes. These acoustic waves and their reflections can cause the prior art vibrating diaphragm sensor to be unstable or inaccurate depending upon the selected chamber geometry, and the present invention derives from an appreciation of these undesired acoustical effects upon the total performance of the vibrating diaphragm gas pressure sensor.